The pathogenic mechanism of Huntington?s disease (HD), a fatal, dominantly inherited neurodegenerative disorder caused by trinucleotide repeat expansion in the huntingtin (HTT) gene, is poorly understood, stymieing therapeutic development. Our overarching goal is to better understand HD pathogenesis and identify targetable pathways for therapeutic intervention by studying genetic modifiers of HD, initially identified by the GeM-HD consortium. The objective of this proposal is to evaluate one HD modifier, ribonucleoside-diphosphate reductase subunit M2 B (RRM2B), by delineating its isoform-specific biological activity on mitochondrial regulation and characterizing its effect on HD pathogenesis. Our hypothesis is that the two SNPs in chr8 GWAS modifier haplotype, rs1037699 and rs5893603, shown to lower RRM2B isoform 2 levels, play a critical role in isoform 2- specific complex(es), altering overall RRM2B activity and HD pathogenesis. Upon completion, this work will validate RRM2B isoform 2 as a modulator of HD pathogenesis and serve as a foundation for novel therapeutic disease-altering strategies as well as providing insight into the fundamental mechanism of HD by: 1) Characterizing isoform-specific RRM2B activity, focusing on its role in mitochondrial regulation, on subcellular localization, stress-response assays, and knockdown experiments in LCLs and human neuronal progenitor cells (hNPCs) and neurons differentiated from our new HTT isogenic series of iPSCs with different CAG sizes (17, 40, 51 and 61); 2) Identifying key cellular protein complexes containing RRM2B isoform 2 in a stress and time dependent manner using APEX-mediated quantitative proteomics; 3) Evaluating potential therapeutic effects by augmenting RRM2B isoform 2 levels in HD patient cells using novel technologies, such as non-coding antisense RNA (SIENUP). Novel techniques and resources include sensitive allele-specific SRM-MS for targeted quantification, recently optimized gene therapy tools, and innovative HD cell model systems, designed to precisely delineate CAG signatures relevant for HD. This proposal will unveil the interface between RRM2B biology and HD pathogenesis, establishing RRM2B as a targetable pathway and potentially revealing other players in the pathogenic cascade. Our expectation is that strategies to upregulate RRM2B isoform 2 level will have a positive effect on HD phenotypes, providing a key candidate for therapeutics. Overall, the positive outcomes of this work will enable us to better understand the effect of genetic modifiers on HD and validate immediate targets for rational drug design, providing additional avenues for future therapeutic strategies.